Rheumatoid arthritis (RA) is an autoimmune disorder that occurs up to four times more frequently in women than men. Evidence suggests that female sex hormones may be at least partially responsible for this trend, in that RA appears to develop during times when these hormone levels fluctuate, such as after giving birth.

Some studies have suggested that breast-feeding may increase the risk of RA, while other studies have suggested that breast-feeding may protect women from the condition.

To further assess this connection, researchers examined data from more than 121,000 female nurses relevant to the age of first menstrual period, number of children, age at first birth, duration of breast-feeding, smoking, body mass index, use of oral contraceptives, incidence of irregular menstruation, and use of hormones following the onset of menopause; 674 women were identified with incident RA.

After adjusting for specific variables, researchers found a significant decrease in the incidence of RA based on the incidence and duration of breast-feeding. Compared to women who did not breast-feed, women who had breast-fed between 12 months and 23 months were 30 percent less likely to be diagnosed with rheumatoid arthritis. Breast-feeding for at least 24 months resulted in a 50 percent reduction in RA risk. The researchers also discovered an increased risk of rheumatoid arthritis in women who had their first menstrual period at age 10 or younger and in young women with "very irregular" menstrual periods.

"In conclusion," note the authors, "we observed that breast-feeding was inversely associated with the risk of RA, with a strong trend for decreasing risk of RA with increasing duration of breast-feeding, and that early age at menarche is positively associated with seropositive RA. In addition, we identified a novel risk factor, irregular menstrual cycles, that increased the risk of subsequent RA ... These findings suggest avenues for further research into the hormonal mechanisms involved in RA, because the complex relationships between RA and reproductive hormones clearly warrant further study."